The present invention relates to an offshore apparatus for carrying out operations in an offshore well. Such apparatus comprises underwater well equipment, a floating platform and a marine riser extending upwards from the underwater well equipment towards the platform.
The marine riser or conductor is used as a means for entering the well with equipment for carrying out operations in the well, such as for carrying out drilling operations or work-over operations. Also, when carrying out drilling operations through the riser, the riser is used as a return path for drilling liquid. The riser may extend between the platform and an underwater wellhead positioned near the bottom of the body of water. In an alternative arrangement, the riser extends between a conductor casing cemented in the sea bottom (or ocean bottom) and an underwater wellhead that is positioned below the water level at a distance that is relatively small with respect to the depth of the body of water at the location where the submerged well is situated. Both arrangements are known. In the first arrangement, the marine riser is removed after completion of the well. In the second arrangement, the marine riser is not removed after ending the drilling operations, but remains as an integral part of the well after completion thereof. An advantage of the underwater wellhead arranged on the upper end of the upstanding marine riser is that the wellhead is at a shallow depth below the water level and can easily be visited by divers for control or repair of the wellhead.
In particular when carrying out operations in submerged wells situated in seas of great depths or in oceans, the weight of the marine riser that has to be supported by the floating platform or by a separate float is extremely large. To reduce this weight, it has already been proposed to mount buoyancy means on the sections of the marine riser.
Each such section consists of a large-diameter open-ended metal tube with coupling means at both ends thereof for coupling the tube to similar tubes of other sections. The buoyancy means may consist of metal containers filled with gas, or of foam plastic bodies of semi-cylindrical shape that are clamped around the tube. The foam plastic bodies may contain gas-filled plastic spheres.